Photoresists are polymers which change their physical character when exposed to the electromagnetic energy at a frequency to which they are sensitive. For example, a photoresist soluble in a particular fluid may, after exposure to a selected source of electromagnetic energy and subsequent development, be insoluble in the same fluid, i.e. the developer. The opposite or schism reaction is equally common.
Photoresists have been used by the semiconductor industry for many years to make optical masks used in the manufacture of semiconductor devices and in particular integrated circuits. The precision and quality of the mask determines the quality of and the density with which circuit components can be packed onto a semiconductor wafer. Briefly, the mask is made by coating a substrate with a photoresist, exposing the photoresist to a selected pattern of energy, developing the photoresist, and etching an underlying substrate to obtain the mask. In a negative acting photoresist, that portion of the photoresist which had been exposed to electromagnetic energy is insoluble in the developer and protects the surface on which it is coated during a subsequent etching step. When a positive acting photoresist is used, it is the portion of the photoresist which is exposed to electromagnetic energy which is soluble in the developer. The resulting mask is most often used in contact with a semiconductor wafer. Alternatively, the same effect may be obtained by using a photoresist coating directly on the semiconductor.
To date, silver halide emulsions have generally been used by the semiconductor industry to make masks because they are relatively sensitive to available energy sources and can be exposed "on-the-fly", that is, without mechanically stopping the emulsion during the exposure time, which may be as short as one to five microseconds. This is very important because a typical mask may require a plurality of exposures to form a pattern; and the number of exposures may be as great as 200,000.
Emulsions, however, have several inherent disadvantages, the primary one being an undesirably low resolution. Therefore, searches are constantly being conducted for systems which could provide higher resolution than that available with emulsions.
Photoresists have generally been regarded by the semiconductor industry as superior to emulsions. Photoresists are generally regarded as having significantly greater resolution than emulsions and have less tendency to produce defects in the final product. Unfortunately, however, photoresists require almost three orders of magnitude more energy than do silver halide emulsions. For this reason, it has hitherto been required that equipment using photoresists stop during exposure.
It is therefore an object of this invention to provide an "on-the-fly" exposure system which can properly expose a photoresist during continuous movement of the resist and at the same time maintain high resolution. Other objects of this invention include providing an "on-the-fly" exposure system for photoresists which is reliable, simple to operate, efficient, and in which exposure times of less than about 250 nanoseconds are obtained. Further objects of the invention include providing an "on-the-fly" exposure system for photoresists having uniform spatial energy distribution in an object plane of the system.